Feature request: repo-local install option that keeps state inside the project

## What I'd love

An install mode where gstack lives entirely inside one project directory, so `rm -rf ` is enough to clean it up. I'm not asking to change the default — just to have a supported path for users who want gstack scoped to a single repo.

## Why this would help

Two small frictions when using gstack in one repo only:

1. **Skill discovery is global.** With the default install, `~/.claude/skills/gstack/` symlinks all skills into `~/.claude/skills/`, so every Claude Code session in unrelated repos sees them too. For users who only want gstack in one project, that's extra context they don't need.

2. **Cleanup touches multiple paths.** Even with `./setup --local`, state still lands in `~/.gstack/` (config, security, optional ML models) and `~/Library/Caches/ms-playwright/` (Chromium). It works fine, it's just spread across a few places, so a clean uninstall isn't obvious.

Neither is a bug. Both are reasonable defaults for the global team-mode flow. This is a request for a second supported path for solo / single-repo users.

## What already exists

- `./setup --local` installs skills into `/.claude/skills/` instead of `~/.claude/skills/`. Marked deprecated, but works for problem (1).
- `~/.gstack/` is referenced in many places across the codebase. There's no env knob to redirect it today.
- Playwright supports `PLAYWRIGHT_BROWSERS_PATH` but `setup` doesn't set it.

## What might be enough

I don't think this needs to be a big project. A few smaller pieces, any of which would help on its own:

- Un-deprecate `--local` (or document it as the supported single-repo mode).
- Optional `GSTACK_HOME` env var that redirects `~/.gstack/` to a custom path. Default unchanged.
- Set `PLAYWRIGHT_BROWSERS_PATH` to a repo-local path when `--local` is used.
- A short uninstall doc that lists the paths a user should delete.

Happy with any subset. Even just the doc would be useful.

## Out of scope

Other AI hosts (Codex, Factory, OpenCode) have their own conventions and probably shouldn't change. Repo-local mode staying Claude-only matches `--local` today.